With recent diversification of electronic devices, battery modules in each of which a plurality of batteries with high capacity, high voltage, high power, and high levels of safety are housed in a housing are needed. In particular, to provide a battery or a battery module with a high level of safety, there is a known technique that provides the battery or the battery module with various protective means such as a positive temperature coefficient (PTC) device or a thermal fuse for preventing a temperature rise and/or a protective circuit for detecting a battery internal pressure to cut off current. There is also a known technique that provides a battery module with a control circuit for controlling charge and discharge of a battery in order to prevent the battery from being in an abnormal state (e.g., a thermal runaway state).
However, even with the protective means or the control circuit as described above, a battery under abnormal conditions might be at high temperatures or subjected to blowout of a high-temperature flammable gas from the inside of the battery. In this case, the housing of the battery module in which the battery is housed might be broken, fused, or overheated, or the blown-out flammable gas might be leaked to the outside of the battery module.
To prevent such problems, the following methods are proposed. In one method, in a battery module in which a plurality of batteries are housed in a housing, a gas released from the battery is diffused in the housing to have its temperature and pressure reduced, and then is released to the outside of the housing (see, for example, Patent Document 1). In another method, a bag configured to be expanded to have a duct shape is attached to a cell group formed by connecting a plurality of cells each having a safety valve that allows a gas to be released when the internal pressure of the cell exceeds a predetermined level. When a large amount of gas is generated, the bag is expanded to form a duct, and then a gas from the cell is released to the outside, thereby reducing the pressure of the released gas (see Patent Document 2).
The foregoing methods are intended to release a generated gas. Another important means concerning safety is prevention of induction of abnormality in other batteries in a battery module when a battery comes to be in an abnormal state. For example, Patent Document 3 proposes the following battery assembly. In this battery assembly, to effectively prevent thermal runaway of a secondary battery from inducing thermal runaway of another secondary battery in the battery assembly, a plurality of secondary batteries are oriented in parallel with each other and arranged adjacent to each other to be housed in an outer case, and a cooling air duct for cooling the secondary batteries with cooling air is placed in the outer case. In addition, a plastic thermal-runaway prevention wall is provided between adjacent ones of the secondary batteries, and is integrally formed with thermally conductive cylinders through which the secondary batteries are inserted such that the thermal-runaway prevention wall is part of the thermally conductive cylinders. The thermally conductive cylinders have heat dissipation regions where the surfaces of the inserted secondary batteries are partially exposed in the cooling air duct so that parts of the secondary batteries exposed in the heat dissipation regions are cooled by cooling air supplied through the cooling air duct.